This invention relates to exhaust gas recirculation systems for motor vehicle combustion engines and particularly such systems utilizing a differential between exhaust back pressure and a control pressure across a flow restricting orifice in the EGR conduit to control the flow of recirculated exhaust gas.
A standard production arrangement for such a system is an exhaust gas recirculation conduit with a flow restricting orifice and an EGR valve downstream therefrom to define a chamber therebetween characterized by a control pressure. Apparatus is provided for comparing the control pressure with a constant pressure reference slightly above atmospheric pressure and controlling the EGR valve to maintain the control pressure constant at the reference value. Since the exhaust back pressure on the upstream side of the orifice varies substantially with the square of engine airflow and the flow through the orifice varies substantially with the square root of the pressure differential across it, the flow through the orifice varies substantially with engine airflow; and the proportion of recirculated exhaust gas to induction air is substantially constant above idle.
It may be found advantageous, however, to be able to vary the proportion of recirculated exhaust gas to induction air in the engine. If the system were modified so that a pressure transducer provided an electrical signal representative of the control pressure and electronic signal processing circuitry controlled the EGR valve in response to the control pressure signal and one or more engine operating variables such as manifold absolute pressure or coolant temperature, a finer control of engine operation would be obtained with possible benefits in the areas of emissions, efficiency of operation or vehicle drivability.
In addition, such a system could allow the use of a control pressure below atmospheric for a greater pressure differential across the orifice which, when used with a smaller orifice size, would provide more accurate control of low exhaust gas recirculation flows while still providing for maximum flow.
However, the use of a pressure transducer to generate an electrical signal may provide a difficulty in controlling exhaust gas recirculation flow at low flow rates. It is generally required that exhaust gas recirculation flow be zero at engine idle, so that the control pressure signal must be equal to a reference voltage representative of exhaust back pressure at idle. A typical pressure transducer, however, is an absolute pressure transducer and may not be sufficiently accurate to guarantee no EGR flow at idle. In addition, the characteristics of the engine or exhaust system which determine exhaust back pressure may change over time, thus producing a different exhaust back pressure at idle and an inaccurate exhaust gas recirculation flow.